The present invention relates to a rotary-wing aircraft, and more particularly to a variable speed rotary wing transmission gearbox system that allows operations at different ratios between the main rotor speed while maintaining an independently variable tail rotor speed.
The forward airspeed of a conventional rotary wing aircraft is limited by a number of factors. Among these is the tendency of the retreating blade to stall at high forward airspeeds. As the forward airspeed increases, the airflow velocity across the retreating blade slows such that the blade may approach a stall condition. In contrast, the airflow velocity across the advancing blade increases with increasing forward speed. Dissymmetry of lift is thereby generated as forward air speed increases.
This dissymmetry of lift may create an unstable condition if not equalized across the advancing and retreating sectors of the rotor disc. Typically, blade flapping and feathering are utilized to substantially equalize the lift. However, as forward airspeed increases beyond a given point for a given rotor rpm, the flapping and feathering action eventually becomes inadequate to maintain substantial equality of lift over the rotor disc. At this point, reverse airflow across the retreating sector creates negative lift and, depending on the forward speed, creates a stalling or negative lift condition that travels outwardly across the blade as airspeed increases.
Conventional main rotor systems must be operated at airspeeds lower than those which cause reverse airflow across a substantial part of the retreating blade and at an rpm low enough to alleviate any potential compressibility Mach number problems at the tip of the advancing blade. This has effectively limited forward airspeeds of conventional helicopters to approximately 180 knots.
Various rotor systems have been proposed which provide variable rotor speed. These systems, however, while changing the speed of the main rotor, also proportionately change the speed of the tail rotor as rotary wing aircraft tail rotors are mechanically linked to the main rotor such that the speeds of each are proportionally related. This prevents conventional rotary wing aircraft from benefiting from decreased rotor speed because the reduced tail rotor speed unacceptably reduces yaw control, so that the desired main rotor speed is not achievable. Similarly, at high forward flight speeds, it is desirable to reduce the tail rotor speed, so that the noise emissions form the rotorcraft are reduced. The tail rotor noise reduction is desirable for operation in populated and congested areas, but is often not possible because flight performance factors prevent a reduction in main rotor speed concurrent with the desirable tail rotor speed reduction.